In imaging methods as e.g. electro(photo)graphy, magnetography, ionography, etc. a latent image is formed that is developed by attraction of so called toner particles. In DEP the so called toner particles are image-wise deposited on a substrate. Toner particles are basically polymeric particles comprising a polymeric resin as main component and various ingredients mixed with said toner resin. Apart from colourless toners, which are used e.g. for finishing function, the toner particles comprise at least one black and/or colouring substances, e.g., coloured pigment.
In the different imaging methods, described above, the toner particles can be present in a liquid or in a dry developer composition.
In most cases the use of dry developer compositions is preferred. The main advantage of using a dry developer composition resides in the absence of the need to eliminate the liquid phase after development. The avoidance of the need to evacuate (mainly organic) liquids is desirable both from an economical standpoint and from an ecological standpoint.
After development of the latent image (in e.g. electro(photo)graphy, magnetography, ionography, etc.) the developed image is transferred to a substrate. In DEP (direct electrostatic printing) the toner image is directly deposited on the substrate.
The visible image, on this substrate, of electrostatically or magnetically attracted toner particles is not permanent and has to be fixed by causing the toner particles to adhere to each other and the substrate by softening or fusing them followed by cooling. Normally fixing proceeds on more or less porous paper by causing or forcing the softened or fused toner mass to penetrate into the surface irregularities of the paper.
There are different types of fusing processes used for fusing a toner powder image to its support. Some are based upon fixation primarily on fusing by heat, others are based on softening by solvent vapours, or by the application of cold flow at high pressure in ambient conditions of temperature. In the fusing processes based on heat, two major types should be considered, the "non-contact" fusing process and the "contact" fusing process. In the non-contact fusing process there is no direct contact of the toner image with a solid heating body.
In "contact" fusing the support carrying the non-fixed toner image is conveyed through the nip formed by a heating roller also called fuser roller and another roller backing the support and functioning as pressure exerting roller, called pressure roller. This roller may be heated to some extent so as to avoid strong loss of heat within the copying cycle. The last mentioned fusing process has been employed widely in low-speed as well as high-speed fusing systems, since a remarkably high thermal efficiency is obtained because the surface of the heating roller is pressed against the toner image surface of the sheet to be fixed. This fusing system has to be monitored carefully in that when the fuser roller provides too much thermal energy to the toner and paper, the toner will melt to a point where its melt cohesion and melt viscosity is so low that "splitting" will occur, and some of the toner is transferred to the fuser roller from where the toner stain may be transferred in a next copying cycle on a next copy sheet whereon it may be deposited; such phenomenon is called "hot-offset", and requires appropriate cleaning. In order to avoid these phenomena an external release agent, e.g., silicone oil, wetting the fuser roller can be used. The application of an external liquid release agent represents an extra consumable and requires apparatus adaptation making it more expensive. Tee release agent will inevitably also transfer to the copy paper and may produce prints having a fatty touch and gloss unevenness due to the presence of oil.
Therefore it is preferred to use in the toner particles, designed to be fixed in a "contact" process, special resins and/or special additives for minimising the need of external release agent or for totally avoiding the use of such an agent.
Several proposition have been made in the art. All of these propositions do, to a larger or smaller extent, overcome the problems with fixing of toner particles in "contact" fusing processes.
It has been disclosed in EP-A-276 147 to add long chain aliphatic alcohols as a wax component to the toner particles for avoiding the problems cited above. In that disclosure it is suggested that, when such an alcohol is added, any ordinary toner resin can be used. Also in JP-A-5967554 the addition of long chain compounds to toner particles has been disclosed.
In U.S. Pat. No. 5,344,737 it has been disclosed to add a component with formula H.sub.3 C--(--CH.sub.2).sub.n --(--O--CH.sub.2 --CH.sub.2).sub.m --OH, with 30.ltoreq.n.ltoreq.50 and 3.ltoreq.m.ltoreq.16 to a toner composition for avoiding the hot-offset phenomenon.
It has also be proposed, in e.g., DE-A-195 20 580, to use polyesters as toner resin that comprise moieties derived from a long chain aliphatic dicarboxylic acid (e.g. eicosanedicarboxylic acid) or that contain long chain mono-carboxylic acid. Also in U.S. Pat. No. 5,578,409 the use of polyester toner resins modified by long chain aliphatic acids or alcohols are described. In EP-A-712 881 the modification of an amorphous polymer by two different long chain aliphatic acids or alcohols has been described.
In EP-A-298 279 it is disclosed to blend multiphase polyorganosiloxane block or graft condensation copolymers in the toner resin. In EP-A-740 217 the modification of a polymer for use in toner particles by a reacting a polymer containing free hydroxyl or acid groups with specified polysiloxanes has been disclosed.
In EP-A-716 351 toner particles are disclosed comprising a high softening point polyester, a low softening point polyester and a long-chain alkyl compound selected from the group consisting of long chain alcohols with 23 to 252 carbon atoms and long chain acid with 22 to 251 carbon atoms. Also toner particles are disclosed comprising as toner resin a mixture of high softening point polyester, a low softening point polyester and a polyester modified by reacting it with a long-chain alkyl compound selected from the group consisting of long chain alcohols with 23 to 252 carbon atoms and long chain acid with 22 to 251 carbon atoms. Toner particles with such toner resin are very resistant to hot-offset.
The disclosures above make it possible to produce dry toner particles with acceptable to good hot-offset properties (i.e. do show very low hot-offset). Toner particles need to have other properties together with the hot-offset properties. During use toner particles are also exposed to severe mechanical stresses, e.g. during mixing, transport trough the devices, by doctor blades regulating the thickness of a toner layer, etc. When toner particles are used in full-colour development the particles must have a very good fluidity during fixing for good interflow of the four colours (Y,M,C,K) and still show good hot-offset properties. A similar good melt fluidity is essential when the grey scale (tonal range) in a black and white electrostatographic image fixed to a final substrate, is extended by realising the necessary different shades of grey with the superposition of toner particles comprising different amounts of black pigment as disclosed in EP-A-768 577. In such image an undesirably high surface relief, making the image very sensitive to scratches can be present, unless there is a very good interflow of the toner particles when fused.
Therefore there is still a need for toner particles that combine good hot-offset properties even with a very low amount of external release agent, and that at the same time are strong enough to withstand the mechanical stresses and have good fluidity when molten.